The present invention relates to a display unit suitable for a display device used for the purpose of reading documents (so-called electronic paper) and its driving method.
In these years, in connection with popularization of the network, documents which were previously distributed in the form of printed materials have been delivered in the form of so-called electronic documents. Further, books and journals are also increasingly provided in the form of so-called electronic publishing. A conventional technique used to read such information is to read the information from a CRT (cathode ray tube) or a liquid crystal display of the computer. However, it is pointed out that it is impossible to use a luminescent type display such as the CRT for long hours of reading and the like since significant fatigue is incurred based on ergonomic reasons. It is said that a non-emissive type display such as the liquid crystal display is not also suitable for reading due to flicker specific to a fluorescent tube. Further, in either case, there is a problem that a reading place is limited to where the computer is installed.
In these years, though a reflective liquid crystal display which uses no back light has been practically used, a reflectance in the case of the liquid crystal is in the range from 30 to 40%. These figures mean significantly bad visibility compared to a reflectance of the printed papers (reflectance of 75% for OA sheets and paperback books; and reflectance of 52% for newspapers). In addition, since fatigue is easy to be incurred due to dazzle by a light reflector or the like, it is impossible to use this reflective LCD for long hours of reading.
Therefore, in order to solve these problems, so-called paperlike displays or electronic papers have been developed. Display methods used for them include an electrophoretic migration method, bicolor ball display method, an electrochromic method and the like. In a display with the electrophoretic migration method (electrophoretic image display: EPID), a white pigment, a black toner and the like are layered on the electrode by the action of electric field. A display with the bicolor ball display method (twisted ball display: TBD) comprises a sphere whose half is colored in white and whose another half is colored in black, and utilizes revolutions by the action of electric field. However, since in both methods, a clearance allowing fluids to gain entry is required and closest packing is impossible, high contrast is hard to be obtained. Further, there is a problem that the practical writing speed (within 1 sec) cannot be obtained unless a drive voltage is 100 V or more. Compared to the displays using these display methods, a display with the electrochromic method (electrochromic display: ECD) is superior to the displays with the foregoing methods in terms of high contrast, and already used practically as a display for, for example, photochromic glasses and timekeepers.
However, as for the electrochromic display, in the case where characters and images are displayed by combining fine picture elements with a simple matrix drive method, there is the danger that its display quality is lowered since its contrast is uniformised due to cross talks between picture elements. Therefore, it is said that an active matrix drive method which arranges active devices such as a transistor for every picture element is desirable. For example, conventionally, an electrochromic display layer is formed on a glass substrate (TFT substrate) wherein thin film transistors (TFT) for every picture element and wiring electrodes or the like are formed. However, in such conventional construction, there is a problem that the display qualities such as luminance and contrast are lowered since the electrochromic display layer is observed from the TFT substrate side so that areas occupied by the TFT and the wiring electrode and the like become shadow.
In light of the foregoing problems, it is an object of the invention to provide a display unit using the electrochromic method which can provide high quality displays and its driving method.